Electronic fetal monitors or Cardio-Toco-Graphs (CTGs) normally consist of a base unit having a thermal printer, a display and multiple sensor elements for measuring vital parameters, such as uterine activity and fetal heartbeat.
Today's fetal monitors use ultrasound Doppler technology for non-invasive acquisition and recording of the fetal heart rate during gestation and labor. As the mechanical contraction of the fetal heart muscle leads to periodic signal patterns in the ultrasound reflection, the period of the patterns is used by fetal monitors to determine the fetal heart rate. U.S. Pat. No. 4,984,576 describes such a device.